Ocean wave power plants of different designs are well known examples of alternative power sources compared to the more traditional power sources in prior art. However, there are very few commercial successfully installations of ocean power plants. The ocean power plants are preferably installed in parts of the ocean providing a steady condition of waves. This implies that preferable locations are the areas of the ocean with harsh weather conditions. This implies that an ocean wave power plant needs to be a durable and strong construction which increases the cost of building the installation and also often the cost of maintaining the installation.
Therefore, the efficiency of the energy production of the wave power plant is of outmost importance. Even though the functioning of a wave power plant is simple to understand for a person skilled in the art it has proved to be a challenge to improve the efficiency of such installations. The cost of the installation, expected maintenance costs etc. must be compared with the probable production outcome of energy, and the energy production must be economically competitive compared with the more traditional power sources providing energy for the market to be able to be regarded as a true alternative power source.
Improving the economy of ocean wave power plants implies that the installations should be cheaper to build and install, and at the same time be able to withstand environmental conditions. Further, the maintenance cost should be lowered and the efficiency of converting wave motions into for example electric energy should be improved. Improving and/or reducing complexity of the technical design of ocean wave power plants does not only improve the economy of ocean wave power plants, but it is also a significant contribution to the emerging field of environmental friendly sustainable technologies for the future.
U.S. Pat. No. 5,359,229 discloses an apparatus that converts wave motion to electrical energy comprising a series of conversion units being interconnected thereby providing continuous rotation of a drive shaft being connected to an electrical generator. Each conversion unit comprises a pylon having a lower portion submerged beneath the surface of a body of water and a top portion extending above the surface of the water. The pylon is held in a fixed position relative to the surface of the water by anchoring the pylon to the floor of the body of water. Attached to the pylon is a float which rises and falls with the rise and fall of waves on the surface of the body of water. The float has a generally spherical exterior and an internal cavity. Ballast such as water is contained within the internal cavity to provide weight to the float. The float further has a central opening through its vertical axis. Mounted within the central opening is a central guide means having a guide sleeve and a plurality of bearings secured to the guide sleeve. The central guide means allows the float to be telescopically fitted around the pylon. The float is thus guided so that it will slide up and down the pylon in a direction parallel to the vertical axis of the pylon. The fixed position of the pylon by the anchoring makes the design vulnerable to harsh weather conditions and the pylon must be able to resist strong forces due to possible huge waves. Even though waves can wash over the installation the plurality of floats will in combination when all are lifted simultaneous have a combined buoyancy force that can tear one or more pylons apart.
U.S. Pat. No. 6,935,808 describes a breakwater for dissipating ocean wave energy and/or for converting such energy into electrical power. The breakwater presented is said to be easier and less expensive to build than existing solutions, which can be constructed in one location and then towed to a desired location and installed there. In one aspect the invention is directed to an apparatus for dissipating waves in the ocean that includes a base anchored to the ocean floor. A tower extends up from the base, with a panel being pivotally attached to the top of the tower, so as to be capable of rocking back and forth. A buoyant element is disposed at the rear edge of the panel, and the panel is configured such that the rear edge of the panel remains above the surface of the ocean and the front edge remains in the ocean when the panel is in its normal state. To facilitate a breakwater that can be more easily installed than a conventional breakwater, the base has variable buoyancy that can be altered by pumping air into the base or venting air out of it. The base includes a plurality of cells having open bottoms into which the air may be pumped and from which the air may be vented. As a result, the base typically will be capable of being manufactured relatively easily and inexpensively. However, the design is intended only for shallow water close to beaches and the design with a defined length of the arm with two opposite located floats makes it only operate properly at certain ocean wave frequencies. If the opposite located floats are lifted or lowered simultaneously by the wave motion, the arm will not move.
US 2783022 from Feb. 26, 1957 by A. Salzer disclose an ocean wave power plant comprising a float resting on the surface of the ocean. Waves respectively lift or lower the float. This movement of the float is transferred via a shaft connected in one end to the float and in the other end to a rack and pinion gear providing a rotational movement of a shaft connected to the pinion gear. The rotational movement of the shaft is therefore correlated with the movement up or down of the float which implies a bidirectional rotation of the shaft back and forth. The disclosed design comprises a deck providing a support for the installation. The position of the deck above the level of the ocean surface may be adjusted. However, the connection point of the shaft to the top surface of the float is subject to strong forces from wave motions and lateral force components from the wave motions may tend to provide ware and tear of the shaft connection of the rack and pinion gear.
U.S. Pat. No. 4,672,222 from Jun. 9, 1987 by P. Foerds Ames comprises a submergible wave power plant installation comprising tubular members approximately forming edge elements of a tetrahedral frustum, and a buoyancy element supported by further tubular members fixed to the bottom part of the installation. The design is self stabilizing, can withstand harsh weather conditions, is modular, and comprises independently operative point absorbers with respective drive mechanisms and electric generators producing electric power from wave motions on a surface of a body of water. The modular design of this ocean wave power plant enable adjacent positioning of the respective modules side by side, wherein the electric power generated in each respective module is summed together and outputted as coming from one power source only. However, the design provides an implicit constraint on the size of the floating body 54 as depicted in FIG. 1 of the disclosure. This limits the amount of energy that can be taken out of the waves form one embodiment of the design. The ability to provide an interconnected plurality of modules, wherein each respective module produces electric energy will of course increase the power output from an installation according to this disclosure. However, the installation tends to be very large covering a substantial part of the ocean surface. Therefore, the cost is high and maintenance is a problem in an interconnected system when a module that is surrounded by other modules need service.
PCT/RS2007/000015 from Aug. 13, 2007 by Mile Dragic disclose a design providing conversion of linear motion up and down of a floating body resting on a body of water, wherein the conversion of the linear motion is provided for by an electric linear induction system or by converting the linear motion into a rotational motion driving an electric generator, for example. A floating body is connected either with a fixed rod or shaft, or a flexible transmission member (wire) to a point on or below the barycentre of the floating body, and in the other end to a generator system producing electric energy when the floating body is lifted up or lowered down by the wave motions. However, the inventor of the present invention has realised that even though the teaching of this patent application provides a significant improvement over prior art, the question of providing a simpler design remains. For example, in this disclosure the support structure comprises a horizontal top beam connected to vertical side beams resting for example on the sea bed. The size of the floating body dictates the possible power output, and hence the size of the support structure, for example the length of the top beam must be increased to allow a certain size of a floating body (or energy output). This may imply a costly design of for example the top beam to provide a stable design that can withstand the size and weight of the floating body, different weather conditions, and at the same time deliver on target for the power production.
There exist some examples in prior art providing teaching about how to convert bi-directional movement of a shaft into unidirectional rotation of a shaft, for example. It is known how to transform the movements back and forth of a piston, for example in an engine for a car. However, these prior art engine solutions requires for example that a rod connected to a piston in the engine can move back and forth in a direction perpendicular to the direction of the movements back and forth of the piston to be able to turn a cam shaft in the engine into a unidirectional rotation. If this additional freedom of motion is constrained, this solution of transforming the piston movement into a unidirectional rotational motion of a shaft is difficult to achieve.
The teaching of U.S. Pat. No. 4,145,885 from Nov. 23, 1977 by Solell disclose a design comprising freewheel devices, gears and chains to combine a first rotation direction of a first shaft and a second rotation direction of a second shaft into a unidirectional rotation of a third shaft. The first rotation direction could be provided for by the movement of a float upwards while the second rotation direction could be provided for when a float moves downwards, for example. However, it is well known by a person skilled in the art that any gear and shaft connection provides a sort of friction in a mechanical system, which in this case provides a loss or decrease of possible power output from an ocean wave power plant. In the theory of power transfer it is well known that the coefficient of efficiency for gear pair is typically 98% and from a chain pair the efficiency is typically 97%, i.e. 1% of wasted energy per pair if a design cannot omit chains. The teaching of U.S. Pat. No. 4,145,885 comprises an installation of a wave power plant at sea wherein a shaft is connected between a supporting deck and the sea bottom. A floating body is arranged to move up and down along this fixed shaft. In this manner vertical force components cannot move the floating body from side to side.
Further, it is obvious that any design that reduces the number of gears that are necessary to use in an ocean wave power plant actually increases the efficiency of the power production itself. In this cited disclosure there is a combination of chains and gears that in itself adds an additional typical 3% to 4% loss of energy as known to a person skilled in the art. Further, in wave power plants shafts etc. are subject to variable speeds due to variable wave conditions. These variations can be abrupt and therefore damage on different parts of a wave power plant may appear as known in prior art, for example. Therefore, it is further obvious that any reduction of gears, choice of technology in the transfer mechanism of energy etc. directly influence cost of production of the installation, maintenance costs and stability of the installation during use of the installation, and may provide an increase of produced power which significantly adds to the profitability of an installation of this kind.
The technical challenge of converting a bidirectional movement of a transmission member interconnecting a float with a mechanism transferring energy of the waves, for example by providing a unidirectional rotation of a shaft, is mainly related to the fact that the length of a stroke of the transmission shaft up or down is strongly variable and are in fact directly related to the amplitude of the ocean waves. Therefore, the use of a cam shaft as known from motor engines is for example difficult to use as readily understood by a person skilled in the art. The use of freewheel devices, gears, chains etc. is known remedies for solving this technical challenge. However, the possible large amplitudes of the waves and the corresponding strong forces make these designs very complicated. The consequence is not that such designs will not function, but that there might be a significant loss of power in the conversion chain due to the number of parts, size of parts etc. It is also a design challenge that the amplitude of the ocean waves might be small. This implies that small amounts of wave energy should preferably be able to be converted by the mechanism in use. This implies that the loss in the conversion chain must be low. The ability to utilize small wave amplitudes is of outmost importance for an ocean power plant to be regarded as a sustainable alternative power source.
When a floating body of an ocean wave power plant is lifted by wave amplitudes that are increasing, it is actually the action of the water itself that is lifted in the wave that is picked up by the floating body. When the floating body is lowered when wave amplitudes are reduced, it is actually the weight of the floating body itself that is providing a drive of the conversion chain since the floating body actually is falling down. It is readily understood that a sufficient weight of the floating body is necessary to achieve an efficient conversion of energy. In prior art it is common to use a large sized body for the float, ref. PCT/RS2007/000015. However, it is a challenge to meet the requirement of providing both buoyancy and weight. When waves lift the floating body upwards it is the buoyancy of the body that provides the weight (the weight of the water) and therefore any torque on an input shaft of a connected generator. This is best achieved with a huge light weight body as known to a person skilled in the art. When the floating body falls downwards it is the weight of the floating body that drives the machinery. However, the increased weight of the floating body may make the floating body subject to damages when experiencing slamming. Slamming is a well known problem in ship design and off shore design. It is possible that a part of a bottom surface of a floating body leaps out of the water due to the wave motions. When the floating body falls down again the bottom surface of the floating body will hit the surface of the water. This impact can provide damages to the installation and the floating body itself. Therefore, safety issues provides that if a floating body leaps out of water the water inside the floating body should be emptied to mitigate the effect of possible slamming.
Therefore it is a need for an improved design of a floating body transferring wave energy in an ocean wave power plant.
According to another aspect of the present invention, a further optimization of energy conversion of a wave power plant may be accomplished by providing a synchronization (or resonance condition) of the movement up and down of a floating body with the frequency of the wave system on the surface of the water the floating body is resting on. In the article “Modelling of hydraulic performance and wave energy extraction by a point absorber in heave” by M. Vantorre et. al. published in Applied Ocean Research 26 (2004) 61-71, it is disclosed theoretical calculations illustrating how a resonant wave power system provides a significant increased extraction of energy. However, there is no indication how to provide a technical solution providing this kind of optimization of energy extraction.
According to an example of embodiment of the present invention, a flywheel is arranged such that the flywheel rotates in a respective direction correlated with a direction of movement respectively up or down of a transmission member connected to a floating body of the ocean power plant. The inertia of the flywheel will then provide a delay of the movement when the floating body turns its direction of rotation. For example, when the floating body is lifted upwards the inertia of the flywheel provided for by the rotation in a direction correlated with the movement upwards of the transmission member, will hold back the floating body a short time interval when the wave lifting the floating body starts to fall downwards again. The movement downwards will of course force the flywheel to rotate in an opposite direction. The inertia of the flywheel will then delay this change of rotational direction. The same situation occurs when the floating body is at its lowest position and starts to be lifted again by the waves. The effect of this delay is to provide a synchronization of the movement of the wave system on the water surface with the natural frequency of the wave power plant system, wherein the weight of the flywheel directly is correlated with the necessary weight.
It is an aspect of the present invention to combine a supporting structural design of an ocean wave power plant that provides a simplification of the support structure, with an optimized wave energy conversion chain and an adapted design of a floating body that can be used in embodiments of the structural design according to the present invention.
It is further an aspect of the present invention to provide an optimized and economical feasible method for deployment of the ocean wave power plant on an ocean sea bed.
Hence, an improved ocean wave power plant would be advantageous, and in particular a more efficient and/or reliable ocean wave power plant would be advantageous.